The present invention relates to a color correction device for applying a color correction to a color image read by an image reading apparatus such as a color scanner.
There is known a color copier for reading an original image by a color scanner including a color CCD (charge coupled device) and forming a color image on a copy sheet based on the read image data.
In such a color copier, the read image is output as image data of three primary colors of R (red), G (green) and B (blue) from the color scanner. The image data of three primary colors are converted into image data of their complementary colors of C (cyan), M (magenta) and Y (yellow), and are then subjected to color corrections defined by masking equations shown in equations (1): EQU D.sub.C' =A.sub.11 .multidot.D.sub.C +A.sub.12 .multidot.D.sub.M +A.sub.13 .multidot.D.sub.Y EQU D.sub.M' =A.sub.21 .multidot.D.sub.C +A.sub.22 .multidot.D.sub.M +A.sub.23 .multidot.D.sub.Y EQU D.sub.Y' =A.sub.31 .multidot.D.sub.C +A.sub.32 .multidot.D.sub.M +A.sub.33 .multidot.D.sub.Y (1).
The above color corrections are performed to improve color reproducibility of the color image formed on the copy sheet by correcting lacking and/or unnecessary color components according to actual spectral characteristics of toners since toners of C, M, Y do not have ideal spectral characteristics.
The color correction in accordance with the masking equations (1) is briefly described taking a color of magenta as an example.
FIG. 9 is a graph showing an example of the spectral characteristic of the magenta toner. In the case that the magenta toner has an ideal spectral characteristic, all rays in a bleu wavelength region .lambda. B and a red wavelength region A R are reflected, and rays in a green wavelength region .lambda. G are completely absorbed as shown in dotted lines in FIG. 9. However, the spectral characteristic of the actual magenta toner is one wherein: the rays in the blue wavelength region .lambda. B and the red wavelength region .lambda. R are partially absorbed and the rays in the green wavelength region .lambda. G are partially reflected.
Accordingly, even if the input image data is pure magenta, the color of magenta image formed on the copy sheet may lack components in the blue wavelength region .lambda. B and the red wavelength region .lambda. R (components in a region A1 of FIG. 9) and may have redundant components in the green wavelength region .lambda. G (components in a region A2 of FIG. 9). Thus, pure magenta cannot be reproduced.
In view of this, the input image data of C-, M-, Y-components are corrected so that the color of the magenta image formed on the copy sheet maximally contains color components of pure magenta. This correction is applied to compensate for the lacking components in the blue wavelength region .lambda. B and the red wavelength region .lambda. R and to reduce the redundant components in the green wavelength region .lambda. G by mixing the image data of C-, M-, Y-components at a specified ratio. Mathematically, this correction is generally expressed by the above masking equations.
It should be appreciated that the correction coefficients A.sub.ij (i=1, 2, 3, j=1, 2, 3) of the masking equations are theoretically or empirically set according to the spectral characteristics of the toners and of the color filters of the color scanner.
In the above color copier, in order to make a black reproducibility of gray satisfactory, a color correction called UCR (under color reduction) is applied.
The UCR is such that, as shown in FIGS. 10A and 10B, a density level corresponding substantially to the half of the minimum density level of the image data of C-, M-, Y-components (hereinafter, "correction level") is subtracted from the respective density levels of the image data of C-, M-, Y-components, an image data of black (BK) components having this correction level is generated, and the generated image data of BK-components is added to the image data of C-, M- and Y-components.
In the example of FIGS. 10A and 10B, since the density level DC of the image data of C-components is at minimum, the density levels D.sub.C, D.sub.M, D.sub.Y of the image data of C-, M- and Y-components are corrected to D.sub.C (=D.sub.C -D.sub.C /2), D.sub.M (=D.sub.M -D.sub.C /2), D.sub.Y, (D.sub.Y -D.sub.C /2), respectively, and then the image data of the BK-components having the same density level as D.sub.C /2 is added to the image data of C-, M-, Y-components.
The known color correction in accordance with the masking equations is applied to correct the color deviations caused by the spectral characteristics of the actual toners and output gradation characteristics, but not to correct color deviations caused by the spectral characteristics of the color filters of the color scanner. Thus, in the case that the color deviation occurs while the image data is read by the color scanner, the color correction in accordance with the masking equations is applied based on the image data of C-, M-, Y-components having experienced the color deviations. Therefore, the colors of the color image formed on the copy sheet is different from the colors of the original image.
Particularly, since unnecessary color components are included in the image data read by the color scanner, if a masking correction is performed based on the read image data of C-, M-, Y-components (or R-, G-, B-components), unnecessary color components are unnecessarily added depending upon the color. As a result, the saturation of colors formed on the recording sheet is lower than that of the colors of the original, thereby giving a sense of incongruity in terms of color reproducibility.
This undesirable color reproducibility is described in detail, taking pure blue as an example. Color filters of RGB color specification system adopted in color scanners generally have spectral characteristics as shown in FIG. 11, the spectral characteristic of each color filter has a wider range of transmission than an ideal spectral characteristic (indicated by dotted line in FIG. 11), and a bottom portion thereof spread into the neighboring color regions. Thus, strictly speaking, the image data read by the color scanner has spectral characteristics different from those of the colors of the original.
In the case that a pure blue original image is read by the color scanner, if the color filters are ideal, B-components are output at high values from the color scanner while no G- and R-components are output therefrom. However, in reality, B-components are output at low values. Thus, if the image data of R-, G-, B-components output from the color scanner are converted into image data of C-, M-, Y-components, it results in many unnecessary Y-components in addition to C- and M-components.
Since an ideal blue image data does not include the image data of Y-components, if the color correction is performed in accordance with the masking equations (1), there can be obtained an image data of Y-components having a suitable density level D.sub.Y. However, if the blue image data includes unnecessary Y-components, the density level D.sub.Y, of the image data after the color correction is higher than a suitable value. As a result, the saturation of a blue toner image is reduced as compared to the original color, making it darker.
In order to improve the disadvantage in terms of the blue color reproducibility, it may be possible to set the correction coefficients A.sub.ij so as to suppress the conversion value D.sub.Y, of the image data of Y-components in the masking equations. However, this makes, for example, the conversion value D.sub.Y, of the image data of Y-components of pure green smaller than necessary, causing a disadvantageous green color reproducibility.
Accordingly, it is difficult to correct the color deviations of the image read by the color scanner only by the color correction in accordance with the masking equations.
In a .gamma.-correction, a .gamma. conversion value of low density black is generally suppressed so as not to make an image as a whole dark. A .gamma. conversion value of gray is suppressed since the density levels of C-, M-, Y-components are reduced by the UCR. Thus, if the color correction in accordance with the masking equations is applied to gray in a similar manner as it is applied to the chromatic colors, the density level of gray becomes higher than a suitable value, thereby making the reproduced gray lighter than the original gray which is a disadvantage of color reproducibility peculiar to gray.